1. Field of the Invention
The present invention relates to a communication line isolator (which is referred to as, for example, a surge protective device (hereinafter referred to as “SPD”) for LAN) arranged so as to intervene in a communication line in order to protect protection target devices corresponding to communication devices including an optical network unit (hereinafter referred to as “ONU”), a home gateway (hereinafter referred to as “HGW”), a personal computer (hereinafter referred to as “PC”) and a modem, which are connected to a communication line such as a local area network (hereinafter referred to as “LAN”) line. In particular, the present invention relates to an isolation-type communication line isolator having high pressure resistance performance (e.g. isolation-type LAN SPD).
2. Description of the Related Art
In order to protect communication devices corresponding to protection target devices from an abnormal voltage or abnormal current such as a lightning surge which enters from a power wire, communication wire or grounding wire, the SPD is installed in each wire. SPD's are broadly classified into two types (i.e. current release type and isolation type) as described below.
As a current-release-type SPD in the related art, for example, there are known a current-release-type SPD for power disclosed in Japanese Patent Laid-Open No. 2008-206263 and a current-release-type SPD for communication disclosed in Japanese Patent Laid-Open No. 11-341677. These current-release-type SPD's are formed with an arrester, varistor and so on, and these components are grounded. As an operation, by causing the SPD to accept an abnormal current and release the abnormal current to the ground side, the abnormal current is prevented from entering the communication device side.
Also, as an isolation-type SPD in the related art, for example, there are known an isolation type SPD for power disclosed in Japanese Patent Laid-Open No. 2005-151705 and an isolation type SPD for communication disclosed in Japanese Patent Laid-Open No. 10-64740. These isolation-type SPD's are formed with an isolation voltage inverter (i.e. isolation transformer), and, although there are many SPD types that are grounded by setting an electrostatic shield to the isolation transformer, there are SPD types that are not grounded. As an operation, by causing the isolation transformer to intervene in a line of a power wire or communication wire, an isolation unit is formed in the line, and therefore an abnormal current does not enter the communication device side as long as the isolation transformer is not destroyed.
When the isolation-type SPD is explained, an isolation transformer used in this isolation-type SPD has a parasitic capacitance between a primary winding and a secondary winding. Therefore, even if the primary side corresponding to the power wire side or the communication wire side and the secondary side corresponding to the power device side or communication device side are isolated using an isolation transformer, part of an abnormal current such as a lightning surge which enters from the primary winding side of the isolation transformer passes to the secondary winding side of the isolation transformer by capacitive coupling by the parasitic capacitance.
To cope with such a pending problem, like the isolation-type SPD's disclosed in Japanese Patent Laid-Open No. 2005-151705 and Japanese Patent Laid-Open No. 10-64740 of the related art, it is desirable to basically set an electrostatic shield between the primary winding side and the secondary winding side of an isolation transformer and fasten this electrostatic shield to the ground. By this means, it is possible to suppress an abnormal current flowed from the primary winding side to the secondary winding side (i.e. it is possible to extremely lower a so-called surge change ratio) and therefore it is possible to reliably protect a power device or communication device from an abnormal current or abnormal voltage. Also, there is a method of preventing an abnormal current or abnormal voltage from intervening in the power device side or communication device side by setting a release-type SPD before the first winding side of the isolation transformer or after the secondary winding side of the isolation transformer and fastening a ground terminal of this release-type SPD to the ground.
Next, an explanation is given to a LAN SPD which is an “isolation type” and “does not require grounding” in the related art.
In recent years, regarding a communication line corresponding to a communication wire, a LAN line becomes common. Communication devices such as an ONU, an HGW and a PC connected to the LAN line include, for example, a LAN connector connected to the LAN line, a pulse voltage inverter (i.e. pulse transformer) connected to this LAN connector and a LAN controller that is connected to this pulse transformer and transmits and receives pulse signals. To protect such a communication device from an abnormal voltage or abnormal current such as a lightning surge, the LAN SPD as disclosed in Japanese Patent Laid-Open No. 2011-10085 of the related art is also installed between the communication device and the LAN line.
For example, in a communication device installed in an average house or building's office, since a ground terminal is often installed in a predetermined limited place, there are many cases where it is difficult to lay a grounding wire from the LAN SPD toward the ground terminal. Therefore, as the LAN SPD used in an average house or building's office, there are many cases where an SPD which is an “isolation type” and “does not require grounding” is required. This kind of SPD is disclosed in, for example, Japanese Patent Laid-Open No. 2008-136303 of the related art.
However, the isolation-type LAN SPD which is one of communication line isolators in the related art has following problems (a) and (b).
(a) Regarding Cooperation Problem
A withstand voltage value of an isolation-type LAN SPD using an isolation transformer in the related art is often designed to around 5 kV. This is because of considering the reality that an abnormal voltage of a lightning surge which enters a LAN line is often equal to or below 5 kV. Further, when an excessive voltage of around 5 kV is applied to the primary side of the LAN SPD, although a voltage is caused on the secondary side of the LAN SPD by capacitive coupling of an isolation transformer, the voltage caused on the secondary side is around 1 kV at a maximum. A communication device connected to the secondary side of the LAN SPD normally has a structure to withstand a voltage of around 1 kV.
Meanwhile, according to the latest studies and field investigation report in recent years, a case is reported where a lightning surge of around 13 kV with an impulse waveform of 10/700 μs (i.e. a lightning surge with a waveform which: reaches the maximum voltage of 13 kV at the timing a time of 10 μs passes from the rising start timing; after that, gradually attenuates; and, at the timing a time of 700 μs passes from the rising start timing, attenuates up to 7.5 kV, which is a half voltage of the maximum voltage of 13 kV) occurs. Therefore, there is a need to improve a withstand voltage of the LAN SPD up to around 13 kV.
To improve the withstand voltage of the LAN SPD itself, the design of an isolation transformer system (e.g. shape) has to be changed.
However, simply, in a case where the withstand voltage performance of the LAN SPD is improved to around 13 kV, the following harmful effect may occur.
For example, when an excessive voltage of around 10 kV is applied to the primary side of the LAN SPD, although a voltage is caused on the secondary side of the LAN SPD by capacitive coupling of an isolation transformer, the voltage caused on the secondary side is several kV (this voltage is larger than 1 kV, the withstand voltage of the communication device on the secondary side) and therefore a communication device on the secondary side may be damaged. In other words, when an excessive voltage of 10 kV which the LAN SPD can withstand is caused, the LAN SPD itself may not be damaged but the communication device on the secondary side may be damaged.
Therefore, it is requested to develop an LAN SPD that not only improves the withstand voltage of the LAN SPD simply but also does not damage the communication device on the secondary side even if the withstand voltage of the LAN SPD is improved.
(b) Regarding Chassis of LAN SPD
In a case where the withstand voltage performance of a LAN SPD is improved up to around 13 kV and a lightning surge of around 20 kV over the withstand voltage performance of the LAN SPD is applied to the LAN SPD, an isolation transformer inside the LAN SPD is subjected to breakdown (i.e. aerial discharge between the primary wire side and the secondary wire side of the isolation transformer), thereby flowing out an excessive abnormal current from the primary wire side to the secondary wire side of the isolation transformer. At this time, in a chassis housing the LAN SPD, an inner pressure is rapidly increased by thermal expansion of air due to an abnormal current.
Here, for example, if the chassis of the LAN SPD having high pressure resistance performance is formed with an isolation member such as a synthetic resin as disclosed in the related art, it is not possible to withstand a rapid increase of chassis inner pressure at the time an abnormal current occurs, and the chassis may burst swiftly. Further, when the chassis bursts, fragments of the chassis fly off and therefore a risk to users is concerned.
Therefore, it can be considered to form the chassis with metal members so as not to burst the chassis of the LAN SPD in a case where a lightning surge over the withstand voltage performance of the LAN SPD is applied to the LAN SPD. However, in the case of the chassis made of metal members, since the chassis is a conductor, it is necessary to make an isolation distance between the chassis and the SPD inner circuit larger than that of the chassis made of isolation members, and thereby the LAN SPD itself becomes large and its cost increases.
Therefore, it is requested to develop a small LAN SPD that not only improves the withstand voltage of the LAN SPD simply but also does not burst a chassis made of isolation members even if the withstand voltage of the LAN SPD is improved.